A relay protection device is an important component of a power system, and plays a very important role in guaranteeing safety operation of the system. It removes faulty equipment when the system is faulty, and contributes to safety operation of the system. However, inaccurate actions about relay protection cause a huge damage on the system.
Current differential protection is widely applied, by virtue of a simple and reliable principle thereof, to protection of a high voltage transmission line, a bus and master electrical equipment. A fundamental operating principle of current differential protection refers to: comprehensively judging, based on a Kirchhoffs current law, whether an internal fault occurs according to a current of each of sections of protected equipment. Thus, the transformation precision of a CT will become a key of influencing precision of the differential protection. As the capacity of the power system increases, when the system is faulty, a fault current is very high, thereby making it possible to cause a phenomenon of CT saturation.
Two sides of a transmission line based on a double-bus connection mode are provided with only one CT, and when the line is externally faulty, currents at two sides can form a differential and brake current to identify whether an internal fault occurs. At present, two switch CTs are mounted at a 3/2 connection side of a transmission line based on a 3/2 connection mode, and a line protection device is connected to a sum current of currents of two switches in engineering. Under an extreme situation, when a fault of a bus occurs during a period for charging a single-side of the line, a fault current flows to a faulty bus from a fault-free bus, i.e., a current passes through the line which is supposed to be current-free in normal situation. However, when one of the switch CTs is saturated, the line will sense the fault current, which may cause a maloperation of differential protection. At present, there is not a method capable of solving the problem of a maloperation of differential protection of a transmission line caused by saturation of a switch CT in a 3/2 connection mode in the art.